This invention relates to radar, and more particularly to Doppler radar, sometimes known as MTI, for moving target indicator. Radars of this type respond only to moving targets. Echoes or returns from stationary targets return to the radar set with an unchanged carrier frequency whereas moving targets will return a Doppler-shifted carrier. The Doppler shift is positive or negative depending on whether the target is moving radially toward or away from the radar set. Radars of this type have been used to detect and measure the speed of moving aircraft for many years. These aircraft Doppler radars have usually relied on skin tracking of their targets for obtaining the desired information. Skin tracking occurs when the radar utilizes echoes reflected by the metal skin of the aircraft. Friendly aircraft are often equipped with transponders which enhance the echo by generating a return signal in response to the incident radar wave. None of these prior art Doppler radars are capable of discriminating between different types of aircraft under observation in the way that the present invention does.
The present invention comprises circuitry which can be connected to or made integral with Doppler radars intended for use against aircraft, and which can discriminate between different types of helicopters and between helicopters and fixed wing aircraft by means of the changes of target echoes of these targets.
The circuitry responds to the periodic changes in the radar reflectivity or cross-section of the helicopter rotors and the airplane propellers as they rotate. The resultant modulation of the target echoes will have a frequency which is characteristic of the type of aircraft under observation, and the circuitry is designed to respond to these different characteristics and thus discriminate between the different aircraft on this basis.